/*
* Syncookies implementation for the Linux kernel
*
* Copyright (C) 1997 Andi Kleen
* Based on ideas by D.J.Bernstein and Eric Schenk.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version
* 2 of the License, or (at your option) any later version.
*
* $Id: syncookies.c,v 1.18 2002/02/01 22:01:04 davem Exp $
*
* Missing: IPv6 support.
*/
#include <linux/tcp.h>
#include <linux/slab.h>
#include <linux/random.h>
#include <linux/cryptohash.h>
#include <linux/kernel.h>
#include <net/tcp.h>
extern int sysctl_tcp_syncookies;
static __u32 syncookie_secret[2][16-3+SHA_DIGEST_WORDS];
static __init int init_syncookies(void)
{
get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
return 0;
}
module_init(init_syncookies);
#define COOKIEBITS 24 /* Upper bits store count */
#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
static u32 cookie_hash(u32 saddr, u32 daddr, u32 sport, u32 dport,
u32 count, int c)
{
__u32 tmp[16 + 5 + SHA_WORKSPACE_WORDS];
memcpy(tmp + 3, syncookie_secret[c], sizeof(syncookie_secret[c]));
tmp[0] = saddr;
tmp[1] = daddr;
tmp[2] = (sport << 16) + dport;
tmp[3] = count;
sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
return tmp[17];
}
static __u32 secure_tcp_syn_cookie(__u32 saddr, __u32 daddr, __u16 sport,
__u16 dport, __u32 sseq, __u32 count,
__u32 data)
{
/*
* Compute the secure sequence number.
* The output should be:
* HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
* + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
* Where sseq is their sequence number and count increases every
* minute by 1.
* As an extra hack, we add a small "data" value that encodes the
* MSS into the second hash value.
*/
return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
sseq + (count << COOKIEBITS) +
((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
& COOKIEMASK));
}
/*
* This retrieves the small "data" value from the syncookie.
* If the syncookie is bad, the data returned will be out of
* range. This must be checked by the caller.
*
* The count value used to generate the cookie must be within
* "maxdiff" if the current (passed-in) "count". The return value
* is (__u32)-1 if this test fails.
*/
static __u32 check_tcp_syn_cookie(__u32 cookie, __u32 saddr, __u32 daddr,
__u16 sport, __u16 dport, __u32 sseq,
__u32 count, __u32 maxdiff)
{
__u32 diff;
/* Strip away the layers from the cookie */
cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS);
if (diff >= maxdiff)
return (__u32)-1;
return (cookie -
cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
& COOKIEMASK; /* Leaving the data behind */
}
/*
* This table has to be sorted and terminated with (__u16)-1.
* XXX generate a better table.
* Unresolved Issues: HIPPI with a 64k MSS is not well supported.
*/
static __u16 const msstab[] = {
64 - 1,
256 - 1,
512 - 1,
536 - 1,
1024 - 1,
1440 - 1,
1460 - 1,
4312 - 1,
(__u16)-1
};
/* The number doesn't include the -1 terminator */
#define NUM_MSS (ARRAY_SIZE(msstab) - 1)
/*
* Generate a syncookie. mssp points to the mss, which is returned
* rounded down to the value encoded in the cookie.
*/
__u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
{
struct tcp_sock *tp = tcp_sk(sk);
int mssind;
const __u16 mss = *mssp;
tp->last_synq_overflow = jiffies;
/* XXX sort msstab[] by probability? Binary search? */
for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
;
*mssp = msstab[mssind] + 1;
NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESSENT);
return secure_tcp_syn_cookie(skb->nh.iph->saddr, skb->nh.iph->daddr,
skb->h.th->source, skb->h.th->dest,
ntohl(skb->h.th->seq),
jiffies / (HZ * 60), mssind);
}
/*
* This (misnamed) value is the age of syncookie which is permitted.
* Its ideal value should be dependent on TCP_TIMEOUT_INIT and
* sysctl_tcp_retries1. It's a rather complicated formula (exponential
* backoff) to compute at runtime so it's currently hardcoded here.
*/
#define COUNTER_TRIES 4
/*
* Check if a ack sequence number is a valid syncookie.
* Return the decoded mss if it is, or 0 if not.
*/
static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
{
__u32 seq;
__u32 mssind;
seq = ntohl(skb->h.th->seq)-1;
mssind = check_tcp_syn_cookie(cookie,
skb->nh.iph->saddr, skb->nh.iph->daddr,
skb->h.th->source, skb->h.th->dest,
seq, jiffies / (HZ * 60), COUNTER_TRIES);
return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
}
extern struct request_sock_ops tcp_request_sock_ops;
static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
struct request_sock *req,
struct dst_entry *dst)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sock *child;
child = tp->af_specific->syn_recv_sock(sk, skb, req, dst);
if (child)
tcp_acceptq_queue(sk, req, child);
else
reqsk_free(req);
return child;
}
struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
struct ip_options *opt)
{
struct inet_request_sock *ireq;
struct tcp_request_sock *treq;
struct tcp_sock *tp = tcp_sk(sk);
__u32 cookie = ntohl(skb->h.th->ack_seq) - 1;
struct sock *ret = sk;
struct request_sock *req;
int mss;
struct rtable *rt;
__u8 rcv_wscale;
if (!sysctl_tcp_syncookies || !skb->h.th->ack)
goto out;
if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) ||
(mss = cookie_check(skb, cookie)) == 0) {
NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESFAILED);
goto out;
}
NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESRECV);
ret = NULL;
req = reqsk_alloc(&tcp_request_sock_ops); /* for safety */
if (!req)
goto out;
ireq = inet_rsk(req);
treq = tcp_rsk(req);
treq->rcv_isn = htonl(skb->h.th->seq) - 1;
treq->snt_isn = cookie;
req->mss = mss;
ireq->rmt_port = skb->h.th->source;
ireq->loc_addr = skb->nh.iph->daddr;
ireq->rmt_addr = skb->nh.iph->saddr;
ireq->opt = NULL;
/* We throwed the options of the initial SYN away, so we hope
* the ACK carries the same options again (see RFC1122 4.2.3.8)
*/
if (opt && opt->optlen) {
int opt_size = sizeof(struct ip_options) + opt->optlen;
ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) {
kfree(ireq->opt);
ireq->opt = NULL;
}
}
ireq->snd_wscale = ireq->rcv_wscale = ireq->tstamp_ok = 0;
ireq->wscale_ok = ireq->sack_ok = 0;
req->expires = 0UL;
req->retrans = 0;
/*
* We need to lookup the route here to get at the correct
* window size. We should better make sure that the window size
* hasn't changed since we received the original syn, but I see
* no easy way to do this.
*/
{
struct flowi fl = { .nl_u = { .ip4_u =
{ .daddr = ((opt && opt->srr) ?
opt->faddr :
ireq->rmt_addr),
.saddr = ireq->loc_addr,
.tos = RT_CONN_FLAGS(sk) } },
.proto = IPPROTO_TCP,
.uli_u = { .ports =
{ .sport = skb->h.th->dest,
.dport = skb->h.th->source } } };
if (ip_route_output_key(&rt, &fl)) {
reqsk_free(req);
goto out;
}
}
/* Try to redo what tcp_v4_send_synack did. */
req->window_clamp = dst_metric(&rt->u.dst, RTAX_WINDOW);
tcp_select_initial_window(tcp_full_space(sk), req->mss,
&req->rcv_wnd, &req->window_clamp,
0, &rcv_wscale);
/* BTW win scale with syncookies is 0 by definition */
ireq->rcv_wscale = rcv_wscale;
ret = get_cookie_sock(sk, skb, req, &rt->u.dst);
out: return ret;
}
